As part of the medical procedure for implanting an implantable defibrillator within a human patient, it is necessary to determine a defibrillation threshold (DFT) that will be effective for that particular patient. The DFT establishes a minimum value for the initial defibrillation countershock which is to be delivered by the implantable defibrillator in the event of a cardiac arrhythmia. A preferred procedure for establishing a DFT during the implantation of an implantable defibrillator is to induce a fibrillation in the patient's heart (i.e., causing the patient's heart to stop pumping), and then using the implantable defibrillator to deliver a defibrillation countershock of a given energy value in an attempt to defibrillate the heart. If defibrillation at the given value is not successful, a very high energy defibrillation countershock is applied from an external defibrillator in order to resuscitate the patient. This procedure is repeated using increasing values for the defibrillation countershock until the defibrillation countershock delivered by the implantable defibrillator is successful in resuscitating the patient, or until the maximum energy value of the implantable defibrillator is exceeded.
In order to conduct this type of DFT test procedure, it is necessary to induce a fibrillation of the patient's heart. Numerous techniques are known for inducing fibrillation by electrical, chemical or other means. During early implantation procedures for implantable defibrillation, it was common for the implantation procedure to involve a transthoracic operation where the heart was exposed so as to allow defibrillation electrodes, such as patch electrodes, to be secured to the heart. In this type of transthoracic operation, it was relatively simple to induce fibrillation of the heart by applying, for example, a 60 Hertz alternating current (AC) signal directly to the exposed heart. As procedures have evolved for the non-transthoracic implantation of implantable defibrillators, the use of externally applied electrical signals to induce fibrillation have become more complicated and less attractive. Accordingly, techniques have been developed to induce fibrillation using the circuitry and implantable leads of the implantable defibrillator system.
The most common technique for inducing fibrillation using an implantable defibrillator is to overdrive a series of pacing pulses applied to the heart so as to induce fibrillation. An example of this overdrive power technique is described in U.S. Pat. No. 4,705,043. In this technique, the patient's heart rate is established and a set of overdrive pacing pulses are then delivered through the implantable pacing electrodes at a rate faster than the patient's heart rate, hence the term "overdrive". While this technique avoids the problems of trying to incorporate a traditional 60 Hertz AC fibrillation signal within an implantable defibrillator, this technique takes longer to induce fibrillation than the traditional 60 Hertz technique and sometimes fails to successfully induce fibrillation. As a result, the patient's heart is subjected to additional stress than would have otherwise been required for induction of fibrillation using 60 Hertz AC techniques, and the implantation procedure is more complicated when this technique fails to induce fibrillation.
Another technique uses a fibrillation-inducing waveform consisting of a medium energy countershock on the order of 100 volts which is intentionally delivered into the T-waves through a pair of defibrillation electrodes. Normally, the delivery of a countershock into the T-wave is considered to be a complication of cardioversion therapy due to the high risk of inducing fibrillation in this situation. In this technique, however, the "complication" is intentionally utilized for the purpose of intentionally inducing fibrillation in order to test the implantable defibrillator. One example of this type of shock into the T-wave technique is shown in U.S. Pat. No. 5,129,392 issued to Bardy, et al. As with the overdrive pacing technique, the shock into the T-wave technique is not completely reliable. In addition, a countershock of this nature is quite painful in the event the patient is not under anesthesia. Also, because the T-wave is a rather subtle and poorly defined characteristic of an electro-cardiogram signal, it is sometimes difficult to use the T-wave to produce the accurate synchronization necessary for consistent induction of fibrillation.
Another technique for inducing fibrillation using an implantable defibrillator is disclosed in European Patent Application EP 0 589 252 82, which describes a technique for generating a multiphasic fibrillation-inducing pulse train as applied to the defibrillation electrodes. The fibrillation-inducing pulse train consists of a series of pulses having a nominal voltage on the order of 15 volts and a pulse width of preferably 1.1 milliseconds with a delay between successive pulses that ranges from 30 to 50 milliseconds. The fibrillation-inducing pulse train is generated by first charging the high voltage capacitor which also delivers the defibrillation countershock to a voltage equal to the nominal voltage and then using additional circuitry and switches separate from the circuitry and switches associated with the delivery of a defibrillation countershock to deliver the fibrillation-inducing pulse train. While this technique may have advantages over the overdrive pacing technique in that fibrillation may be induced more quickly and more consistently, this technique has the disadvantage in that it requires additional circuitry not normally associated with the implantable defibrillator. In addition, the fibrillation which is typically induced using this technique tends to be a relative coarse fibrillation wherein the electrical activity of the heart, even though in fibrillation, still exhibits a relatively high degree of organization and coordination. As such, this type of coarse fibrillation does not provide for an optimal test of the DFT of the implantable defibrillator.
While it is desirable to provide for a system for inducing fibrillation that is part of the implantable defibrillator, the present techniques for inducing fibrillation using an implantable defibrillator are not as effective as the traditional technique of applying an external 60 Hertz AC signal to the exposed heart. Accordingly, it would be advantageous to provide a system for inducing fibrillation using an implantable defibrillator which improved upon the effectiveness of the current techniques without requiring that significant additional circuitry be included within the implantable defibrillator.